The present invention relates to an exposure control device which enables both each independent black dot and each independent white dot on a face to be printed securely when an image is formed by using a laser beam.
In response to a demand for image quality of higher fineness, a printer employing a laser diode has come into use recently.
FIG. 18 shows one example of an image forming portion of this known printer. In the known printer of a type shown in FIG. 18, a thin film member 11 having a peripheral length slightly larger than an outer peripheral length of a developing roller 10 is mounted around an outer periphery of the developing roller 10 such that higher image quality is secured. A photosensitive drum 15 is adapted to be rotated in the direction of the arrow shown in FIG. 18 and a surface of the photosensitive drum 15 is uniformly charged to a predetermined potential by a corona charger 16. When a position on the photosensitive drum 15 has been rotated through a predetermined amount after this charging of the surface of the photosensitive drum 15, a laser beam is irradiated at various positions on the photosensitive drum 15 based on image data and thus, an electrostatic latent image is formed on the photosensitive drum 15. Subsequently, when the photosensitive drum 15 has been further rotated, toner transported by the thin film member 11 is adsorbed to a low potential portion of the photosensitive drum 15, i.e. a portion on which the laser beam has been irradiated so as to develop the electrostatic latent image into a visible toner image. Thereafter, when the visible toner image has reached a transfer position, the toner image is transferred onto a recording paper sheet 17 by a transfer charger 16 and then, the transferred image is fixed to the recording paper sheet 17 by a fixing device (not shown).
The light emitting state of the above laser beam is controlled as shown in the flow chart of FIG. 19. Initially, a portion (not shown) for controlling the light emitting state of the laser beam reads out image data of one pixel at step S1 and judges at step S2 whether or not the image data is a printing pixel, i.e. a pixel for which a toner image should be formed. In case the read-out pixel is the printing pixel at step S2, a light emitting duty ratio is set to "1" at step S3 as shown in FIG. 20 such that the laser beam is emitted at a predetermined quantity of light during a period corresponding to a width of one pixel. On the contrary, if the read-out pixel is not the printing pixel at step S2, the light emitting duty ratio is set to "0" at step S4 as shown in FIG. 20 such that irradiation of the laser beam is not performed. Then, based on the light emitting duty ratio set at step S3 or S4, the laser beam is emitted so as to perform exposure of the photosensitive drum 15 at step S5. After exposure of the photosensitive drum 15 has been performed, the program flow returns to step S1 at step S6 such that the same processings as described above are performed for the next pixel. The foregoing processings are sequentially performed for all given image data so as to form an image on the printing paper sheet 17, whereby printing is completed.
However, the known printer operated as described above has such a drawback that since emission and nonemission of the laser beam are controlled merely by presence and absence of the image data, it is difficult to print with fidelity both each white dot and each black dot existing independently of one another.
For example, when only one dot is black data in image data of six dots as shown in FIG. 21(C), the quantity of light of the laser is controlled such that the one dot is printed as black with fidelity. In this case, even if three dots should be continuously printed as black in image data of six dots as shown in FIG. 21(A), a printed state is obtained in which an area larger than the three dots becomes black. Meanwhile, even if only one dot should not be independently printed as black in image data of six dots as shown in FIG. 21(B), all the six dots are printed as black.
As will be seen from the distribution of quantity of light shown in FIGS. 21(A) to 21(C), the above mentioned printed states result from the fact that when a light beam approximate to Gaussian distribution is scanned, leaked light of the light beam is slightly irradiated also on an area of nonprinting pixels at an edge portion of an image. In case this leaked light exceeds an illustrated threshold quantity of light, the pixel is printed as black. Therefore, when the light beam exceeding the threshold quantity of light has been irradiated to form dots, the dots are printed as black as shown in FIGS. 21(A) and 21(B) even if the dots are nonprinting image data as shown.
If this leaked light is restrained to such a extent that one independent white dot can be reproduced as shown in FIGS. 22(A) and 22(B), the problem referred to above should be solved. However, on the other hand, if one independent black dot should be printed as shown in FIG. 22(C), the quantity of light of the laser beam does not reach the threshold value or more and thus, such an opposite problem arises that one black dot cannot be printed.